XIV. Magnetic and other Physical Properties of Iron at a High Temperature. 
By John Hopkinson, M.A., D.Sc., F.P.S. 
Received April 16,—Read May 9, 1889. 
[Plates 12 -20.] 
It is well known that for small magnetising forces the magnetisation of iron, nickel, 
and cobalt increases with increase of temperature, but that it diminishes for large 
magnetising forces."" Bauer! has also shown that iron ceases to be magnetic some¬ 
what suddenly, and that the increase of magnetisation for small forces continues to 
near the point at which the magnetism disappears. His experiments were made 
upon a bar which was heated in a furnace and then suspended within a magnetising 
coil and allowed to cool, the observations being made at intervals during cooling. 
This method is inconvenient for the (Calculation of the magnetising forces, and the 
temperature must have been far from uniform through the bar. In my own experi¬ 
ments | on an impure sample of nickel the curve of magnetisation is determined at 
temperatures just below the temperature at which the magnetism disappears, which 
we may appropriately call the critical temperature. 
AuerbachS and CallendarII have shown that the electrical resistance of iron 
II 
increases notably more rapidly than does that of other pure metals. Barrett,* * * § ** ! in 
announcing his discovery ^of recalescence, remarked that the phenomenon probably 
occurred at the critical temperature. Tait investigated the thermo-electric pro¬ 
perties of iron, and found that a notable change occurred at a red heat, and thought 
it probable that this change occurred at the critical temperature. 
It appeared to be very desirable to examine the behaviour of iron with regard to 
magnetism near tlm critical temperature, and to ascertain the critical temperatures 
* Rowland, ‘ Phil. Mag.,’ Nov., 1874. 
t ‘Wiedemann, Annalen,’ vol. 11, 1880. 
X ‘ Roy. Soc. Proc.,’ June, 1888., 
§ ‘Wiedemann, Annalen,’ vol. 5, 1878. 
II ‘ Phil. Trans.,’ A, 1887. 
^ ‘ Phil. Mag.,’ Jan., 1874. 
** • Edinbui'gh Roy. Soc. Trans.,’ Dec., 1879. 
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